Considering the commercial success of MEMS-based accelerometers, MEMS gyroscopes are believed to be the next trending application in the MEMS industry. MEMS gyroscopes offer self-contained rotation measurement of an object. Even after years of development, however, the MEMS gyroscope market is still limited to a small portion of consumer, automotive and low-end military applications. It is believed that what keeps MEMS gyroscopes from a wide range of consumer markets is their high cost/performance ratio. While a low-cost MEMS gyroscope with a compromised, or lower level, performance has proven successful for some applications, at the same time, the lower performance point has kept such gyroscopes from much more potential applications such as, for example, personal navigation or dead reckoning applications.
The application of a MEMS gyroscope to enable miniaturized inertial navigation systems (INS) for personal navigation requires tri-axial rotation sensing. Different types of MEMS yaw gyroscopes have demonstrated promising performance, however, developing high-performance out-of-plane pitch and roll gyroscopes is known to be very challenging.
The performance of a gyroscope can be evaluated through its signal to noise ratio (SNR). The SNR of a gyroscope can be increased by noise reduction and improvement in rate sensitivity. One approach to achieving increased sensitivity is through mode-matched operation where the drive and sense modes having the exact same resonance frequency. When mode-matched, the Coriolis force excites the sense mode at its resonance frequency, leading to a Q-amplified sense response. However, perfect mode-matching may not be possible due to cross-coupling of resonance modes, i.e., quadrature error, caused by fabrication non-idealities. Quadrature error breaks the eigenvalue-degeneracy of the equations of motion, resulting in a veering phenomenon that appears as a minimum obtainable frequency-split between the drive and sense modes of a gyroscope. In addition, quadrature error provides a path through which drive-loop noise is carried to the sense mode and becomes a major noise contributor in the sense output signal.
Considering both effects, quadrature error significantly degrades the output SNR and attempts have been made to minimize it in order to achieve better performance in a MEMS gyroscope. One known approach is electrostatic quadrature tuning in a mode-matched yaw gyroscope. However, in out-of-plane gyroscopes that are used for pitch and roll detection, and fabricated using wafer level processing and DRIE techniques, mode alignment or quadrature tuning electrodes are typically unavailable, which makes quadrature error the biggest obstacle in realizing high-performance pitch and roll gyroscopes.
In addition, the relatively high cost of high-performance MEMS gyroscopes is a result of their complexity and limited manufacturability. As above, in high-performance gyroscopes, mode-matched operation is required for a large SNR. Based on their operation frequency, mode-matched gyroscopes can be sorted into low frequency gyroscopes and high frequency gyroscopes. The sensitivity and noise performance parameters of low frequency gyroscopes rely on a large proof-mass and a large drive amplitude. As a result, low frequency gyroscopes are large in size and sensitive to shocks and linear accelerations. On the other hand, high frequency gyroscopes take advantage of high frequency resonant modes with high quality factors. High-frequency gyroscopes are smaller in size and have better shock resistance, which makes them ideal for a variety of applications, however, high frequency gyroscopes often involve complex fabrication processes. In addition, small fabrication imperfections can cause problems like frequency mismatch and cross-coupling (quadrature), which highly limit their yield and performance. This is true especially for pitch and roll gyroscopes, where both planar and thickness variations play important roles.
Accordingly, a need exists for a MEMS gyroscope design that results in improved performance over known MEMS gyroscopes and that can be manufactured at a cost that makes the device cost-competitive for high volume consumer electronics products.